Tonight the Saints player knocked on at exactly 79:30 and the game never restarted but I've seen refs stop the clock sometimes for this even before the scrum has formed especially late on, anyone know what the exact rule is regarding scrums?

One team has to pack down fully before the clock can be stopped. Usually with such a short space of time to go, and no chance of the losing team to win, they'll usually just let to clock wind down.

If one team rushes to form a scrum, the clock will be stopped when they have formed a 6 man scrum and the scrum half is in position.

If the team that formed the scrum then break up and the other team form the scrum, time will be called back on.

Gus Gould has suggested several times that in the last 10 minutes of each half the clock should stop automatically for scrums, line droputs etc.